[Illustration: FIG. 91.--Types of _Helobiæ_. _A_, inflorescence of arrowhead (_Sagittaria_), with a single female flower, × ½ (_Alismaceæ_). _B_, section through the gynœcium, showing the numerous single carpels, × 3. _C_, a ripe fruit, × 3. _D_, a male flower, × 1. _E_, a single stamen, × 3. _F_, a leaf of _Sagittaria variabilis_, × ⅙. _G_, ditch-moss (_Elodea_), with a female flower (_fl._), × ½. (_Hydrocharideæ_). _H_, the flower, × 2. _an._ the rudimentary stamens. _st._ the stigma. _I_, cross-section of the ovary, × 4. _J_, male inflorescence of eel-grass (_Vallisneria_), × 1. _K_, a single expanded male flower, × 12. _st._ the stamen. _L_, a female flower, × 1. _gy._ the stigma.] The last family is the _Hydrocharideæ_. They are submersed aquatics, or a few of them with long-stalked, floating leaves. Two forms, the ditch-moss (_Elodea_) (Fig. 91, _G_, _I_) and eel-grass (_Vallisneria_) are very common in stagnant or slow-running water. In both of these the plants are completely submersed, but there is a special arrangement for bringing the flowers to the surface of the water. Like the arrowhead, the flowers are unisexual, but borne on different plants. The female flowers (_H_, _L_) are comparatively large, especially in _Vallisneria_, and are borne on long stalks, by means of which they reach the surface of the water, where they expand and are ready for pollination. The male flowers (Fig. 91, _J_, _K_) are extremely small and borne, many together, surrounded by a membranous envelope, the whole inflorescence attached by a short stalk. When the flowers are ready to open, they break away from their attachment, and the envelope opens, allowing them to escape, and they immediately rise to the surface where they expand and collect in great numbers about the open female flowers. Sometimes these are so abundant during the flowering period (late in summer) that the surface of the water looks as if flour had been scattered over it. After pollination is effected, the stem of the female flower coils up like a spring, drawing the flower beneath the water where the fruit ripens. The cells of these plants show very beautifully the circulation of the protoplasm, the movement being very marked and continuing for a long time under the microscope. To see this the whole leaf of _Elodea_, or a section of that of _Vallisneria_, may be used. CHAPTER XVII. DICOTYLEDONS.